The present invention relates to an apparatus for the decontamination of soil under environmentally acceptable conditions.
The effective cleanup and disposal of soils which are contaminated with petroleum products and other contaminates have recently become major environmental concerns. As one specific example of the problem, a large number of underground fuel tanks have been found to be leaking and releasing a variety of contaminates into the surrounding soil, and as these tanks are being removed and repaired pursuant to environmental regulations, the volume of contaminated soil has been growing. Many communities no longer permit such soil to be placed in landfills, and thus a need exists for the decontamination of the soils.
In one conventional system for treating and decontaminating such soils, the soils are heated in a rotary drum dryer to a temperature sufficient to volatilize the hydrocarbons and other contaminates, and possibly also oxidize the contaminates. The gas stream is then removed from the drum dryer and delivered to a cyclone separator or baghouse, where the fine dust particles are removed from the gases. Also, the dust particles which are removed by the cyclone separator or baghouse are delivered onto the discharge screw conveyor which conveys the heated soil which has been discharged from the drum dryer to a suitable stockpile or storage area. A system of this general type is disclosed for example in U.S. Pat. No. 4,827,854 to Collette.
It has been recognized that in the above process, the potential exists for some portion of the entrained dust particles which leave the drum dryer with the exhaust gases to remain contaminated, and these contaminated particles are untreated. U.S. Pat. No. 5,176,445 to Mize discloses a decontamination process wherein the exhaust gases and entrained particles are delivered to an incinerator before delivery to the dust separator, and the incinerator acts to oxidize and decontaminate the contaminated dust before separation. However, the cost of an incinerator which is capable of properly treating the dust as well as the exhaust gases, can be significant.
Another proposed solution for ensuring the proper treatment of the airborne dust particles in a soil remediation plant is disclosed in U.S. Pat. No. 4,957,429 to Mendenhall. The Mendenhall patent discloses a rotary drum dryer for heating the soil, and wherein the exhaust gases are fed through a cyclone separator and a baghouse to separate and remove the entrained dust particles from the airstream, and with the gases then being delivered to a furnace which oxidizes the gaseous contaminates. The removed dust particles are returned to the interior of the drum dryer, so that the dust particles are again heated with the contaminated soil in the dryer. The patent states that this process can reduce the concentration of volatilizable organic composition to less than about 100 parts per million in the resulting mixture.
It is believed that the recycling of the dust particles back into the drum dryer as disclosed in the above Mendenhall patent will inevitably permit some of the same dust to be re-entrained in the gas stream in the dryer and again removed to the dust separators, and that the system will eventually become overloaded.
It is accordingly an object of the present invention to provide an apparatus and method for effectively and economically decontaminating petroleum contaminated soils and the like.
It is a more particular object of the present invention to provide an apparatus for decontaminating soils, and which effectively avoids the potential for a portion of the dust particles which are entrained in the exhaust gas stream to be untreated and remain contaminated when the gas stream is released to the atmosphere.